KIEV, Ukraine (CNN) -- The two men vying to become president of Ukraine exchanged attacks in a bare-knuckled debate Monday, six days before they face new elections that will be watched closely in many parts of the world.

Opposition leader Viktor Yushchenko -- whose loss in the rejected November 21 election and mysterious dioxin poisoning have galvanized international attention -- came out swinging against Prime Minister Viktor Yanukovych.

Yushchenko accused Yanukovych of stealing the last election.

International observers reported widespread fraud and irregularities in the race, and the Ukrainian supreme court ultimately ruled it invalid.

Yanukovych has the backing of Russian President Vladimir Putin, who had praised the November 21 result even amid the reports of problems.

Yushchenko skewered his rival over ties, and visits, to Moscow. "We have to make sure the Ukrainian president is not elected in Moscow because these trips to Russia are very humiliating," he said, adding that Yanukovych's trips to Warsaw were troubling as well.

But Yanukovych attacked Yushchenko as a virtual tool of the West.

CNN Senior International Correspondent Walter Rodgers said both men were playing on xenophobia.

Yanukovych also played on Ukrainian fears of instability and chaos, Rodgers said.

Yanukovych argued that he could unify the country, but his rival could not.

"If you think that you're going to win and you're going to become the president you are greatly mistaken," he said in the debate.

"If you win you can only become president of part of Ukraine. I don't want this. I want the two of us to unite Ukraine."

The lengthy debate, with questions posed by a moderator, lasted an hour and 45 minutes.

Both men promised economic and political reforms.

Yushchenko wore an orange tie, the color of his campaign; Yanukovych wore his trademark blue.

Both men called for Sunday's elections to take place without irregularities.

The Bush administration -- which had criticized the November 21 elections and said they could not be recognized -- expressed hope for Sunday's new vote.

U.S. State Department spokesman Richard Boucher said Ukraine has learned lessons from the recent election and made changes that "will help ensure that we have a more open and credible result this time."

He noted that the Ukrainian parliament, the Verkhovna Rada, has undertaken "substantial reform."

Also, he said, thousands of Ukrainian observers would watch the elections, as well teams of foreign observers and members of the Organization of Security and Cooperation in Europe.

"We certainly hope that with all those things in place and the new attitude being expressed in Ukraine, as well as the new attention that's being focused by the whole world on Ukraine, that this round can truly reflect what the Ukrainian people want, and that the results will reflect what the Ukrainian people decide," Boucher said.